1. Field of the Invention
The present invention relates to electrical cables, in particular low-voltage electrical cables having improved fire resistance and water resistance.
2. Description of the Prior Art
Fire-resistant cables including polyolefin layers containing an organic halogenide or other fire retardants are known in the art.
The disadvantages of using halogenated flame retardant compounds, in particular in connection with fabrication of the above cables and their possible corrosive effect on metal parts of the equipment, are known in the art.
PCT WO 99/05688 discloses cables with good fire resistance properties, including a layer of crystalline propylene homopolymer or copolymer blended with a copolymer of ethylene with at least one xcex1-olefin and possibly a diene.
The above compounds contain a natural magnesium hydroxide in quantities which confer fire retardant properties on the cables.
The above application mentions the possible use of an internal insulative layer that is relatively thick compared with the layer containing the fire retardant.
European application EP 0 378 259 discloses high-voltage connecting cables including a metal wire surrounded by a first insulative layer from 0.3 to 1.5 mm thick and including a copolymer of propylene and at least one other alkene, the first insulative jacket being covered with a second insulative jacket that is not crosslinked and is from 1.0 to 3 mm thick, containing polyvinyl chloride or a vinyl chloride copolymer. The second jacket includes fire retardant additives.
GB patent 2 294 801 concerns an electrical cable including a conductor embedded in a hydrophobic material and surrounded by a layer of fire retardant material. The cable is used in the field of telecommunications in particular.
However, we have found that using a covering including a layer containing the fire retardant as described in PCT WO 99/05688 makes the water resistance of the cable unsatisfactory, failing to comply with the relevant requirements. Also, when the thick insulative additional layer is used, the cable has fire resistance problems.
We have found that when the cable is live in the presence of moisture the water tends to penetrate the cable and reduce its insulation, leading to breakdown of the cable.
The invention therefore relates to a low-voltage electrical cable, preferably a single-conductor cable, having improved fire resistance and flame propagation resistance, containing no halogenides and complying with IEC standards, the cable having improved water resistance. The combination also aims to improve the mechanical properties of the cable and its resistance to short circuits.
The cable according to the invention must have a fire resistance complying with the following standards: IEC 332-3c (March 1992), NFEN 50 266-2 section 4 of February 1999, and NFC 32070 section 2 and 2.2 of March 1991, and the additions A1 of July 1992 and A2 of November 1993, and a water resistance such that it resists salt water (10 g/l NaCl) at 60xc2x0 C. at a voltage of 220 V DC for at least ten days, complying with NFC-32-201-2, art. 2.5 of October 1998 and NFC-32-201-1 table 3, section 5 of October 1998.
We have discovered that the above problems can be solved by developing a cable featuring a double layer, the layers having relative thicknesses such that the cable has at one and the same time improved fire resistance properties and improved water resistance properties, in both cases complying with the aforementioned standards.
The invention provides a fire-resistant and water-resistant low-voltage electrical cable including a conductor and a first internal layer to protect it against water based on a polymer compound containing no halogen, crosslinked or not, and a second layer consisting of a blend of a crystalline propylene homopolymer or copolymer and of a copolymer of ethylene and at least one xcex1-olefin, optionally with a diene, and of an agent having fire retardant properties, the ratio of the thicknesses of the outer layer and the internal layer being from 1 to 7.
Depending on the cross section of the conductor, the internal layer preferably has a thickness from 0.05 to 1 mm and the outer layer preferably has a thickness from 0.25 to 2 mm.